In the field of medical treatment, there have been concerns in processing of imaging materials with respect to effluent produced from wet-processing, and recently, reduction of the processing effluent has been strongly demanded in terms of environmental protection and space saving. There has been desired a photothermographic material for photographic use, capable of forming distinct black images exhibiting high sharpness, enabling efficient exposure by means of a laser imager or a laser image setter.
There are known, as such a technique, silver salt photothermographic materials comprising an organic silver salt, light-sensitive silver halide and a reducing agent on a support, as described in “Dry Silver Photographic Materials” (Handbook of Imaging Materials, Marcel Dekker, Inc. page 48, 1991) U.S. Pat. Nos. 3,152,904 and 3,487,075. Such a silver salt photothermographic material, which does not employ any solution type processing chemical, can provide users a simple and environment-friendly system.
In one aspect, this photothermographic material contains, in the image forming layer (light-sensitive layer), a light-sensitive silver halide as a photosensor and a light-insensitive silver salt of an aliphatic carboxylic acid as a silver ion source, and is thermally developed usually at 80° C. or higher by an included reducing agent for silver ions (hereinafter also denoted simply as a reducing agent) to form an image, without performing fixation.
However, such a photothermographic material, in which an organic silver salt and light-sensitive silver halide are contained together with a reducing agent, readily causes fogging, resulting in practical problems. Further, in the process of emulsion making, a conventional silver halide emulsion employs, as a protective colloid, a hydrophilic dispersant such as gelatin, producing problems upon exposure of silver halide emulsion grains to an hydrophobic organic solvent, which causes aggregation or ripening. On the other hand, there are many advantages in the prior art of using hydrophilic gelatin as a protective colloid, for instance, a technique of forming silver halide grains in water, a chemical sensitization technique by use of water-soluble sensitizers and a setting storage technique by use of a hydrophilic gelatin as a protective colloid, and there is not resolved the problem that silver halide grains coagulate in a hydrophobic solvent.
Conventionally, aqueous-dispersed silver halide grains were mixed in the stage of preparing aliphatic acid silver salt grains, producing problems such as loss of development initiating points (lowering of image density) due to ripening/aggregation of silver halide grains and increased fogging (during development and after storage) caused by increased chance of being brought into contact with an aliphatic acid silver salt. Probability of silver halide grain being brought into contact with an aliphatic acid silver salt increases with change in phase from a liquid to a thin layer. Contact probability with the aliphatic acid silver salt was conventionally controlled by lowering the silver density by the matrix binder content in the film. However, lowering the silver density results in reduced thermal development speed, producing problems which hinder rapid access.
Recently, there is an increased necessity for nano-order microparticles in various fields and is noted a technique of dispersing nano-particles. However, there are several hurdles to achieving a technique of dispersing inorganic particles such as silver halide grains in a resinous solvent, because the surface of inorganic microparticles is generally hydrophilic, rendering it difficult to disperse the particles in organic solvents. There was disclosed a technique in which, to disperse hydrophilic inorganic particles in lipophilic solvents, a hydrophobic dispersing agent is attached as a protective colloid through a chemical bond to the surface of hydrophilic inorganic particles to achieve dispersion of the particles in a lipophilic solvent, as described in JP-A No. 5-111631 (hereinafter, the term JP-A refers to Japanese Patent Application Publication).
There is not known in the prior art a technique of dispersing lipophilic particles in which a hydrophobic dispersant is applied to aqueous-dispersible hydrophilic protective colloid particles.
There was disclosed, as an amphiphilic dispersion system, a thermally sensitive polymer of which hydrophilicity/hydrophobicity is reversibly changeable at a phase transition temperature, being a threshold value, as described in JP-A No. 7-276792. However, this technique is thermally reversible, resulting in a defect that variation of modification in water and hydrophobic solvents is greatly limited.
There was disclosed, as a technique of hydrophilic protective colloidal particles covered with a lipophilic polymer also exhibiting hydrophobicity, the use of a polymer obtained by random copolymerization of a hydrophilic monomer and a hydrophobic monomer, as described in JP-A 2005-166696. However, it was difficult to balance hydrophilicity and hydrophobicity in a random copolymer and was also not sufficient as a technique for balancing adsorption onto hydrophilic protective colloidal particles and dispersion in a hydrophobic solvent.
In order to achieve enhancement of both adsorption onto hydrophilic particles and dispersion in a hydrophobic solvent, a block copolymer is more preferable than a random copolymer. It is assumed that formation of a block of a hydrophilic portion and also a block of a hydrophobic portion facilitates display of their functions and also balances hydrophilicity and hydrophobicity.
Production of an amphiphilic block copolymer employed a method of preparing a polymer by use of a polymeric initiator containing a hydrophilic group or a hydrophobic group. Specifically, there were reported some methods for preparing an amphiphilic block copolymer by using a polymeric initiator containing a polyoxyalkylene group, as described in JP-A Nos. 2001-288233, 2002-97236 and 2006-88131. However, there was not reported any example of dispersing highly hydrophilic particles such as silver halide grains using gelatin as a protective colloid in a hydrophobic solvent by use of an amphiphilic block copolymer prepared by allowing a hydrophobic polymer to copolymerize with a polymeric initiator containing a polyoxyalkylene group.